As the cost of copper and copper cable rises and the population of the world increases, it is becoming more important for telephone companies to avoid installing new telephone cables to meet increased demands for service. High costs of new cable, new repeaters, new poles, and new central office switching circuitry would be incurred. In addition, labor costs in installing new cable on new poles and tearing up the streets and highways for new underground routes would drive costs still higher. All these new costs would necessitate rate hikes with the ultimate cost being borne by the telephone subscribers.
Accordingly, telephone utilities have sought ways to stretch their already existing facilities by such techniques as digital time division multiplexing and frequency multiplexing so that larger numbers of telephone users can all share the fixed number of circuits available in the cables already in place. Frequency division multiplexing involves transmitting the outgoing speech signals of a telephone line on one frequency and receiving the incoming speech signals for that line on a different frequency. Each telephone line then operates independently on its own pair of carrier frequencies, which pair of frequencies is different from all the other pairs of frequencies assigned to the other telephones sharing the common link. All the frequencies then are transmitted over the common link with frequency selective filters associated with each telephone such that no interference between the individual conversations occurs.
The multiplexing system of the invention uses multiplexing whereby each conversation is superimposed on the shared span line or assigned a particular time slot on a common circuit in a cable, which time slot is different from the time slots assigned to all the other conversations traveling down the common circuit "simultaneously". The analog or varying amplitude voice channels comprising the transmit and receive portions of each telephone conversation are converted to digital signals comprised of binary words or bytes. Each byte is a predetermined number of bits in length. Each combination of logic ones and zeroes in each byte represents a different amplitude level of the speech signal during a particular sample period. That is, each varying amplitude speech signal is chopped into a number of pieces of the same duration, which duration is called the sampling period. An analog-to-digital converter then converts the amplitude of the speech signal during each sampling period to a digital word in a code which represents that amplitude. For example, if an eight-bit word is used with each bit either a one or a zero, there are 256 distinct combinations which can be made. Each combination represents a specific amplitude level of the speech signal. These data words are then transmitted to the other party over the common circuit during the assigned time slot whereupon circuitry at the other end reconverts the digital words back to analog signals and transmits them to the earpiece of the second telephone. A similar circuit transmits the speech signals from this second telephone back to the earpiece of the first telephone during the same time slot.
Each common link circuit is called a span line, and each span line is comprised of two pairs of wires, one pair for each direction of transmission. Each span line in the disclosed system normally handles sixty-four telephone lines, and there are two span lines in the system, such that one-hundred-and-twenty-eight users can be coupled to the central office over only four pairs of wires. When one span line breaks down, all 128 users are coupled to the remaining span line.
Because span lines cover large distances and cause losses such that signals become weaker as they travel down the line, it is necessary to insert repeater amplifiers in the line to periodically boost the signal levels. Thus, it is advantageous to use digital multiplexing because it is easier for the repeaters to detect binary ones and zeroes than analog signals. As long as the repeaters can distinguish between a one and a zero, the distance of transmission has no effect on received signal quality. Also, identical circuitry is used as opposed to frequency multiplexing where different frequencies require different frequency selective filters.
The present invention pertains generally to the field of digital multiplexed telephone systems and, more particularly, to the circuitry for self-testing the audio channels, A/D and D/A converters, filters, and buffers which make the conversions.